Addition of oxygen in the Ca0.5Sr0.5Ga2S4:Ce phosphor has been found to nearly double the efficiency and shift the Ce+3 4f-5d emission spectrum to a deeper blue. To understand the effects that oxygen has on the Ce+3 4f-5d emission spectrum, we used a self-consistent-field configuration interaction (SCF/CI) model to calculate the electronic spectrum of oxygen-doped SrS:Ce. By replacing the appropriate number of sulfur ligands with oxygen ligands, the SCF/CI model predicted a blue shift for two different cerium-oxygen bond distances that is in very good agreement with the 730 cm(-1) blue shift observed for Ca0.5Sr0.5Ga2S4:Ce. An analysis of atomic orbital electron populations indicates that changes in covalency of the Ce 5d orbitals upon incorporation of smaller O-2 ions causes the observed blue shift. Calculated oscillator strengths indicate that the observed increase in electroluminescent efficiency caused by addition of oxygen is not a result of changes in transition intensities.